The present invention is directed toward medical electrical leads generally, and more particularly toward cardiac pacing leads.
Endocardial pacing leads can generally be divided into two basic groups, depending upon the type of mechanism employed to maintain the electrode on the distal tip of the lead in contact with heart tissue. Leads employing mechanisms which do not penetrate heart tissue are typically referred to as xe2x80x9cpassive fixationxe2x80x9d leads. Such leads are disclosed, for example, in U.S. Pat. No. 3,902,501 issued to Citron. Leads employing mechanisms which penetrate heart tissue are typically referred to as xe2x80x9cactive fixationxe2x80x9d leads. The most common type of active fixation lead employs a helical fixation member which is screwed into heart tissue. Examples of such leads are disclosed in U.S. Pat. No. 4,106,512 issued to Bisping and U.S. Pat. No. 4,217,913, issued to Dutcher. These leads may include advancable fixation helixes, as in the Dutcher and Bisping patents previously cited, or may employ fixation helixes which are inextendable and mechanically coupled to the outer lead body, as disclosed in U.S. Pat. No. 5,076,285 issued to Hess et al and U.S. Pat. No. 5,473,812 issued to Morris et al. A hybrid between the two types may employ a lead with a fixation helix which is rotationally fixed with respect to the lead body, but which can be slid into and out of the distal end of the lead. Such leads are disclosed in U.S. Pat. No. 4,967,766 issued to Bradshaw, U.S. Pat. No. 5,228,455 issued to Barcell.
In both active and passive fixation leads, it is often desirable to employ electrodes in a bipolar configuration, such that located proximal to the tip electrode, is a second electrode, typically taking the form of a conductive ring encircling the lead body. The electrode assembly at the distal tip of the lead and the ring electrode, are typically quite rigid, with the lead body intermediate the tip and ring comprising a tip-ring spacer of some sort carrying a single conductor. Because the lead body proximal to the ring electrode carries two conductors, it is typically stiffer than the portion of the lead intermediate to tip and ring electrodes. The result of this basic structural arrangement is that bending forces applied to the distal portion of the lead, for example due to contraction of the heart, tend to localize bending in the tip-ring spacer region of the lead. With some present designs, bending tends to uneven along the length of the tip-ring spacer, with small radius bends occurring adjacent the proximal end of the tip electrode assembly and/or the distal end of the ring electrode, raising the possibility of stress induced conductor fractures at these points.
The present invention is directed toward providing a lead with an improved tip-ring spacer, optimized to provide a controlled radius of curvature throughout the length of the tip-ring spacer and to avoid areas of high radius of curvature which might otherwise occur. The tip-ring spacer of the present invention accomplishes this goal by means of an internal lumen having a dual-taper configuration. The spacer has a generally uniform outer diameter while the central lumen therethrough has an inner diameter which increases gradually from a first diameter at its proximal end to a second diameter at a point intermediate the proximal distal ends of the spacer, and which gradually tapers distally to a third, reduced diameter adjacent the proximal end of the electrode assembly. In one preferred embodiment of the invention, the tip-ring spacer is formed of a material having a relatively high modulus of elasticity, in order to resist bending stress at the proximal and distal ends of the spacer, and is provided with an insert having a lumen therethrough of a generally uniform diameter, and an outer configuration corresponding to the inner configuration of the lumen through the tip-ring spacer. This insert may be provided by molding or backfilling, and preferably has a lower modulus of elasticity, optimized to balance the overall bending characteristics of the lead in the tip-ring space region to provide for a uniform radius of curvature along the length of the tip-ring spacer. Alternatively, a lower modulus material may be chosen for the spacer, and the dual-tapered lumen may be left unfilled.
The present invention is particularly beneficial in leads employing helical fixation devices, as in such leads, the conductor coupled to the fixation helix is typically a high torque, coil conductor in which the individual coil wires typically have a cross section greater than that which might be employed in a passive fixation lead, which in turn may result in a reduced ability to withstand repeated flexing over a small radius of curvature. However, the invention is also believed to be useful in the context of passive fixation leads as well.